In recent years, with economic progress and social development, modern people pay more attention to their own health and gradually spend more time to do exercise. However, urban life is very busy, it is not easy for most people to do outdoor exercise frequently; for this reason, the fitness industry is rapidly developed, and various types of exercise devices are available in market, thereby providing people to do indoor exercise for different targets, such as improvement of muscular endurance or cardiopulmonary function.
The following takes an exercise device for training muscular endurance as example for illustration. The exercise device usually includes a weight assembly (such as weight discs or bar pieces), and a user can apply force to pull the rope or handle against weight of the weight assembly; when the user raises the weight assembly through the rope or handle, the user's muscle groups are contracted. When the user reduces the force to make the weight assembly move downwardly, the user's muscle groups are relaxed gradually. By rising the weight discs or bar pieces of the weight assembly repeatedly, the user can work muscle groups to contract and relax repeatedly, thereby improving strength and endurance of the muscle groups.
The aforementioned exercise device is popular among fitness enthusiasts because of its simple structure, but it still has obvious drawbacks in actual use. The reason is described below. The conventional exercise device uses weight discs or bar pieces to provide resistance and the user exerts force to overcome the resistance to train muscle, but the maximum forces applied by muscle of the user at different statuses are different. Biceps of the user's arm is taken as example, the maximum force applied by biceps when the user's arm is bent is 1.5 times of that when the user's arm is straightened, that is, the maximum force applied by arm in bent status and straightened status are different. While the user operates the exercise device to repeatedly raise weight discs or bar pieces, the user bents and straights arm continuously, but a number of the weight discs or bar pieces cannot be changed based on the bent or straightened status of the user's arm. If the user selects the weight discs or the bar pieces with weight corresponding to the maximum force of the arm in the bent status, while the user is straightening the arm gradually, the user may strain muscle of arm because the weight discs or the bar pieces are too heavy for the straightened arm.
In order to solve aforementioned problem, the applicant has designed a transmission mechanism applied to a fitness device including a driving wheel and a driven wheel disposed on a main structure thereof, a multi-groove transmission belt wound on the driving wheel and the driven wheel, and a damper is disposed on an axle of the driven wheel. The user can drive a handle on the main structure by hands or legs, to apply force against resistance of the damper for training purpose.
However, the applicant finds a drawback of aforementioned fitness device in use. The fitness device may have unbalance of tension on the multi-groove transmission belt because of the manufacturing or assembly error, or elastic fatigue or misalignment of the multi-groove transmission belt, and the unbalance effect may cause that a plurality of convex sections formed on wheel faces of the driving wheel and the driven wheel cannot be engaged into a plurality of grooves of the multi-groove transmission belt, respectively. Each of the driving wheel and the driven wheel has a plurality of convex sections formed on the wheel face thereof and engaged with the multi-groove transmission belt, but abnormal wear may occur between the multi-groove transmission belt and the driving wheel and the driven when the multi-groove transmission belt is misaligned because of tension unbalance, and the abnormal wear may result in twist of the multi-groove transmission belt, and even the multi-groove transmission belt may be torn off and the fitness device may be damaged. As a result, the company may spend too much in repair and maintenance of the fitness device.
Therefore, what is need is to improve the structure of the aforementioned fitness device, so as to solve the problem that the transmission mechanism of the fitness device may be damaged because of the unbalance of tension on the multi-groove transmission belt caused by the manufacturing or assembly error.